The present invention generally relates to a pneumatic tire and, more particularly, to a bias-ply pneumatic tire suited for use on a motorcycle.
There is known a pneumatic tire suited for use on a motorcycle which comprises, as best shown in FIG. 1 of the accompanying drawings, a tire cover 10 having a tread portion 11, a pair of opposed side walls 12 one on each side of the tread portion 11 and integral with the tread portion 11 and a pair of opposed bead portions 13 integral with the respective side walls 12 and adapted to firmly engage with associated rim flanges 21 of a wheel rim 20, a bias-ply carcass structure 14 embedded in the tire cover 10 and having opposed ends turned up around and anchored to the respective bead portions 13 in the tire cover 10, and an inner lining 15 of a blended rubber material which is impervious to air and which is coated on the inner surface of the tire cover 10. The bias-ply carcass structure 14 is constituted by a plurality of carcass plies of juxtaposed textile cords, which carcass plies are alternately laid down on a bias relative to the midcircumferential plane of the tire.
The pneumatic tire of the construction so far described may be used in practice either with or without an inner tube therein. However, where the pneumatic tire is designed for use without the inner tube, that is, in the form of a tubeless pneumatic tire, the inner lining 15 has a wall thickness larger than that required in the pneumatic tire for use with the inner tube therein. The employment of the larger wall thickness of the inner lining 15 is necessitated to prevent objectionable diffusion of air from within the tire cover 10 and through the side walls, such diffusion of air being objectionable not only from the standpoint of loss of inflation pressure, but also because of a high rate of diffusion of air into the side walls 12 tending to blister and separate the carcass plies from the body material forming the tire cover 10. The inner lining 15 usually extends circumferentially of the tire between the opposed bead portions 13 and extends down to the region of the toes of the respective bead portions 13.
On the other hand, it has often experienced that, when the pneumatic tire in use is, for example, pierced with the inflation pressure consequently reduced, at least one of the opposed bead portions of the tire is displaced in a direction generally transversely of the midcircumferential plane of the tire into a rim well or drop center portion of the wheel rim under the influence of an external lateral force acting on the tire in a direction generally transversely of the midcircumferential plane of the tire. Once this displacement happens, the worst that may happen would be a traffic accident and/or physical damages to a motorcyclist.
In order to avoid the possible displacement of the bead portion 13 into the rim well, an improvement has been made to the design of the wheel rim, an example of which is shown in FIG. 1 of the accompanying drawings. Referring still to FIG. 1, the wheel rim, generally identified by 20, includes a substantially radially inwardly concaved rim well 22, generally flat bead seats 23 one on each side of the rim well 22, a pair of opposed, generally radially outwardly extending rim flanges 21, and a pair of opposed ribs 24 each radially outwardly protruding from one side portion of the corresponding bead seat 23 remote from the rim flange 21 and extending circumferentially of the wheel rim 20. When the tire is mounted on the wheel rim 20, a rim engaging face or base of each of the bead portions 13 is seated on the corresponding bead seat 23 with toe and heel respectively tightly contacting the adjacent rib 24 and the adjacent rim flange 21.
The conventional wheel rim 20 shown in FIG. 1 is so designed that, when one or both of the bead portions 13 of the pneumatic tire tends to be displaced into the rim well 22, the corresponding rib 24 prevents such bead portion 13 from slipping into the rim well 22.
Since the pneumatic tire for particular use on a motorcycle is generally small in size, the effective width RW of the wheel rim 20 for the support of such tire thereon is limited to a relatively small value and the width WW of the rim well 22 is correspondingly limited to a value as required for the effective mounting of the tire on the wheel rim 20. Therefore, if the wall thickness of the inner lining 15 is increased to enable the tire to be used in practice without an inner tube disposed therein, that is, to enable the tire to be used as a tubeless tire, the pneumatic tire having the inner lining 15 of increased wall thickness can no longer be mounted on the wheel rim 20 having the bead seats 23 with a fixed width SW. This means that a particular construction of the wheel rim is required, depending upon which tire is utilized, that is, depending on whether or not an inner tube is utilized in the tire. More specifically, if the wall thickness of the inner lining 15 in the tire designed for use with the inner tube is merely increased to enable such tire to be used in the form of a tubeless tire, the volume of each of the bead portions 13 increases correspondingly to such an extent that the base of the corresponding bead portion 13 can no longer be accommodated within the fixed width SW of the associated bead seat 23 of the wheel rim 20. If the tubeless tire, the bead portions of which have their bases of a width larger than the fixed width SW of the associated bead seats 23 of the wheel rim 20, is mounted on the wheel rim 20 of the construction shown in FIG. 1, there is the increased possibility of the lateral displacement of the bead portion 13 into the rim well 22.
On the other hand, if an attempt is made during the manufacture of the tire to render the width BW of the base of each bead portion to be equal or slightly smaller than the width SW of the corresponding bead seat 23 by reducing the wall thickness of the tire cover 10 and/or the number of carcass plies, the resultant tire would no longer be used as a tubeless tire with the bead portions exhibiting a poor performance. As is well known to those skilled in the art, each of the bead portions of the pneumatic tire in general has a number of tasks, i.e., to anchor the carcass plies to withstand the deformation forces resulting from running the tire, to withstand the inflation pressure, to transmit longitudinal forces and to provide a seal between the tire and the wheel rim. Therefore, where the poor performance of the bead portions is such that a complete seal cannot be obtained between the tire and the wheel rim and/or that the tire cannot withstand the deformation force acting particularly in a direction generally transversely of the midcircumferential plane of the tire, such tire cannot be used in the form of a tubeless tire, that is, without the inner tube.
Yet, it has been well recognized by those skilled in the art that the pneumatic tire having the bias-ply carcass structure is superior to the pneumatic tire having the radial-ply carcass structure in respect of providing a comfortable riding, a rigidity of the side walls, and a stable drivability particularly during cornering. However, the bias-ply pneumatic tire requires by nature a larger number of the carcass plies than that in the radial-ply pneumatic tire. In view of this, the manufacture of the bias-ply pneumatic tire involves some problems peculiar to such bias-ply pneumatic tire. By way of example, not only is the placement of the carcass plies one above the other to form the bias-ply carcass structure complicated and time-consuming, but also it requires a lot of labor and skill in workmanship. In addition, the productivity of the bias-ply pneumatic tire is limited as compared with that of the radial-ply pneumatic tire.